Method for monitoring wavelength-division multiplexed signal

ABSTRACT

A method for monitoring wavelength-division multiplexed (WDM) signal for detecting signal drift of objective signals, including generation of one or more objective signals and a guard signal. The guard signal has a wavelength that is within a range defined by a guard channel. The first and second objective signals and the guard signal are wavelength-division multiplexed to generate a wavelength-division multiplexed signal. The first objective signal, the second objective signal, and the guard signal are assigned to a first multiplexed objective channel, a second multiplexed objective channel, and a multiplexed guard channel, respectively. The wavelength-division multiplexed signal is received by a monitor and then the error rate of the multiplexed guard channel is determined.

FIELD

This description relates generally to detecting signal drift of awavelength-division multiplexed signal by providing and monitoring awavelength-division multiplexed signal.

BACKGROUND

Electromagnetic radiation signals can be undesirably affected by signaldrift. Generally, signal drift, also known as frequency drift, is anunintended offset of a signal's carrier frequency from its nominalfrequency. Because of the inverse relationship between frequency andwavelength, frequency offset can be described using a wavelength of thesignal. There are several causes for signal drift. For example, changesin temperature that can affect components of a system or overallbreakdown of components leading to system failure can cause signaldrift.

Signal drift can be particularly problematic if there are multiplesignals sent at different wavelengths along the same connection. Forexample, optical signals having different carrier frequencies may becommunicated along the same optical fiber. For example, awavelength-division multiplexed signal uses different and discretewavelength channels to communicate multiple sets of data along anoptical fiber. When a signal drifts in a wavelength-division multiplexedsignal, a signal drift of one channel, or wavelength, can cause thatchannel to drift into its adjacent channel causing interference. Foroptical signals, interference is caused by leaking photons from thedrifting channel to another channel. Such interference can causesignificant noise in a channel. Noise causes error rates of a signal orchannel. Generally, it is desirable to minimize error rates of a signalor channel. A prolonged interference or substantial amount ofinterference can lead to unacceptable rise in the error rate. Increasederror rate can lead to the total loss of data on the channel.

Wavelength-division multiplexing is a technique that multiplies signalsso that multiple signals are carried together along a same fiber, eachof the signals separated at different wavelengths or channels on themultiplexed signal. Wavelength-division multiplexing may be used toconstruct various types of networks. A ring topology network and a startopology network are just two of several known network topologies whichcan be created to utilize wavelength-division multiplexing to generate amultiplexed signal. A star topology network has a central hub andvarious nodes connected to the central hub with multiple inputs andoutputs. Each node may generate a signal that is sent to the centralhub, where the multiple signals are wavelength-division multiplexed andthe multiplexed signals are sent back to the individual nodes.Accordingly, each of the nodes may detect the multiplexed signal, whichgenerally allows each individual node to see the signals from all of theother nodes via the channels of the multiplexed signal. A passive starcoupler is a star coupler that does not require any additional power towavelength-division multiplex signals to generate a multiplexed signal.A ring topology may wavelength-division multiplex signals of multiplenodes in a serial fashion, wherein signals from each node arewavelength-division multiplexed at the leg of the ring topology fromnode to node, that can ultimately lead to a complete multiplexed signalat a leg of the ring topology.

For a system that uses multiple signals using multiple wavelengths alongthe same optical fiber, it would be advantageous to be able to detectsignal drift. One possible method is to use a spectrum analyzer. Aspectrum analyzer is used to examine the spectral composition of theoptical waveform. For example, a spectrum analyzer can calculate aFourier transform of a signal, resulting in a waveform in a powerspectrum, wherein different frequency components of the waveform areshown as separate bands or channels over a given frequency range. Poweror magnitude of each frequency component may also be shown in the powerspectrum. However, there are several disadvantages of using a spectrumanalyzer. Spectrum analyzers require expensive equipment and may not beable to be integrated into some systems or solutions. Further, becauseof signal-to-noise ratios, spectrum analyzers use long acquisition timesand signal processing methods such as signal averaging. Further, theNyquist frequency limit also may hinder perfect reconstruction of thesignals from the waveform. To resolve this issue, further steps may berequired, such as using different filters and oversampling. Further, useof these techniques may cause ghost signals, wherein certain frequencycomponents are included in the output but were not part of the originalsignal.

Accordingly, improved methods for detecting signal drift are desirable.

BRIEF SUMMARY

This description relates to a method for monitoring wavelength-divisionmultiplexed (WDM) signal. Embodiments disclosed herein relate to asystem and a method for monitoring a WDM guard signal. Embodimentsdisclosed herein relate to a method for determining signal drift ofobjective signals by providing and monitoring a guard channel of a WDMsignal.

In one embodied method for monitoring a WDM signal, one or more guardsignals are generated, each guard signal including a guard wavelengthλ_(g), and a predetermined guard power amplitude A_(g). The one or moreguard signals are then wavelength-division multiplexed along with asignal, generating a wavelength-division multiplexed signal. Thewavelength-division multiplexed signal includes a multiplexed guardpower amplitude A′_(g). Then the multiplexed guard power amplitudeA′_(g) is detected and an error rate of the multiplexed guard channel isdetermined.

The signal that is wavelength-division multiplexed with the one or moreguard signals may be a wavelength-division multiplexed signal thatincludes at least one power amplitude at a channel; and the guardwavelength λ_(g), being different from the channel.

In another embodied method, a first objective signal and a secondobjective signal are generated, the first objective signal including afirst objective wavelength and the second objective signal including asecond objective wavelength. A guard signal is generated, including aguard wavelength. The guard wavelength is within a range defined by aguard channel. The guard channel includes a range of wavelengths that isbetween the first objective wavelength λ_(o1) and the second objectivewavelength λ_(o2). The first and second objective signals and the guardsignal are wavelength-division multiplexed to generate awavelength-division multiplexed signal. The wavelength-divisionmultiplexed signal includes channels (or wavelengths) at which the datafrom the first and second objective signals and the guard signal areassigned. The first objective signal, the second objective signal, andthe guard signal are assigned to a first multiplexed objective channel,a second multiplexed objective channel, and a multiplexed guard channel,respectively. The wavelength-division multiplexed signal is received bya monitor and then the error rate of the multiplexed guard channel isdetermined.

The determination of the error rate of the multiplexed guard channel caninclude, for example, a method of calculating the bit error rate (BER)of the multiplexed guard channel, or a method of calculating thesignal-to-noise ratio (SNR) of the multiplexed guard channel.

The first objective signal may further include a first objective poweramplitude A_(o1), and the guard signal may further include apredetermined guard power amplitude A_(g), wherein A_(g) is less thanA_(o1).

While it is advantageous to have a low error rate for multiplexedobjective signals, it is desirable to have a measurable amount of errorrate for the multiplexed guard signals. Accordingly, methods may beemployed so that for the multiplexed guard signals, error rate would beenhanced. One possible method is to have a lower multiplexed guard poweramplitude A_(g). Preferably, the multiplexed guard power amplitude A_(g)is less than or equal to 50% of A_(o1).

The determination of the error rate of the multiplexed guard channel mayinclude a method of comparing the guard power amplitude A_(g) of theguard signal to the multiplexed guard power amplitude A′_(g) of themultiplexed guard channel, to determine an error guard power ΔA_(g),wherein ΔA_(g) is a mathematical function of A′_(g) and A_(g).

The guard channel may include an action protocol associated to themultiplexed guard channel. Preferably, the guard channel includes anaction protocol associated to each of the multiplexed guard channels. Anaction protocol may include, for example, a predetermined criterionwherein if an error rate at the guard wavelength is determined to meet acertain threshold, then an output is generated according to the actionprotocol. For example, the output may be a recommendation for a systeminspection. Preferably, an output recommending a system inspectionincludes when to perform the system inspection, such as within a certaintime period (e.g. X hours, Y minutes) and/or a fixed determined time(year, month, date, hour, etc.). Preferably, an action protocol mayinclude a predetermined criteria for certain wavelength wherein anoutput is a system malfunction alert indicating that an immediateattention to the system is required and/or a system shutdown isrequired.

In another embodiment, the method for monitoring a WDM signal furthercomprises generating a second guard signal. The second guard signalincludes a second guard wavelength λ_(g2) of the guard channel, whereinthe second guard wavelength λ_(g2) is selected according to a channelhopping protocol. Then generating a second wavelength-divisionmultiplexed signal, wherein the first objective signal, the secondobjective signal, and the second guard signal are wavelength-divisionmultiplexed. The second wavelength-division multiplexed signal includesthe first multiplexed objective channel, the second multiplexedobjective channel, and a second multiplexed guard channel. The secondwavelength-division multiplexed signal is received by a monitor and thena second error rate of the second multiplexed guard channel isdetermined.

Preferably, the channel hopping protocol includes selecting the secondguard wavelength λ_(g2) from a predetermined set of guard channelwavelengths λ₁ to λ_(N) (N≧2).

The channel hopping protocol may include selecting the second guardwavelength λ_(g2) randomly from a predetermined set of guard channelwavelengths λ₁ to λ_(N) (N≧2). The term “randomly” includes a method ofcalling a random number generator from a computer or a computing device.

The channel hopping protocol may include time variably selecting thesecond guard wavelength λ_(g2) from the guard channel. Time variablyselecting a wavelength may include a resulting wavelength from acalculation of a mathematical equation that is a function of the timevariable.

In another embodied method for monitoring a WDM signal, the methodcomprises generating one or more guard signal(s) G_(i), wherein i=1 toN, and N≧1. Each guard signal includes a guard wavelength λ_(i) and apredetermined guard power amplitude A_(i). One or more objectivesignal(s) S_(j) are also generated, wherein j=1 to M, and M≧1. Eachobjective signal includes an objective signal wavelength λ_(j). Then,the guard signal(s) and the objective signal(s) are wavelength-divisionmultiplexed, generating a wavelength-division multiplexed signal,wherein the wavelength-division multiplexed signal includes amultiplexed guard channel for each λ_(i). Then an error rate E_(i) ofthe multiplexed guard channel at channel λ_(i) are determined. Thedetermination of the error rate includes, for example, a method ofcalculating the bit error rate (BER) of the multiplexed guard channeland/or a method of calculating the signal-to-noise ratio (SNR) of themultiplexed guard channel. Other possible methods would be recognized tothose skilled in the art.

A system for monitoring a WDM signal comprises, for example, a firstnode that generates a first objective signal for carrying data, a secondnode that generates a second objective signal for carrying data, and aguard channel monitor that generates a guard signal. The system alsoincludes a central hub configured to receive the first objective signal,the second objective signal, and the guard signal. The central hub isconfigured to generate a multiplexed signal by wavelength-divisionmultiplexing the first objective signal, the second objective signal,and the guard signal. Further, the central hub may be configured to sendthe multiplexed signal to the guard channel monitor, wherein the guardchannel monitor is configured to compare the guard signal to themultiplexed signal at a predetermined wavelength. Comparing the guardsignal to the multiplexed signal includes, for example, determining anerror rate using methods of calculating BER and/or SNR.

Preferably, the central hub is a star coupler, such that the system hasa star topology network. Even more preferably, the star coupler is apassive star coupler.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an illustration of a wavelength-division multiplexedwaveform.

FIG. 2 shows an illustration of a wavelength-division multiplexedwaveform with a detectable signal drift.

FIG. 3 shows an illustration of a wavelength-division multiplexedwaveform with a detectable signal drift.

FIG. 4 shows an illustration of a wavelength-division multiplexedwaveform.

FIG. 5 shows an illustration of a wavelength-division multiplexedwaveform with a detectable signal drift.

FIG. 6 shows an illustration of a wavelength-division multiplexedwaveform with a detectable signal drift.

FIG. 7 shows an illustration of a wavelength-division multiplexedwaveform.

FIG. 8 shows an illustration of a wavelength-division multiplexedwaveform.

FIG. 9A shows an illustration of a first wavelength-division multiplexedwaveform using a channel hopping technique.

FIG. 9B shows an illustration of a second wavelength-divisionmultiplexed waveform using a channel hopping technique.

FIG. 9C shows an illustration of a third wavelength-division multiplexedwaveform using a channel hopping technique.

FIG. 10 shows an example of a system including a star topology network.

DETAILED DESCRIPTION

The term “wavelength-division multiplexing” is defined herein to includefrequency division multiplexing (FDM). The term “signal” is defined toinclude electromagnetic radiation signals. Electromagnetic radiationsignals include, for example, optical signals and radio signals. On awaveform, the term “signal” may also describe portions of the waveformthat carries data. For example, in a wavelength-division multiplexedsignal having a particular waveform having multiple peaks in amplitude,each of the peaks may be described as “signals” wherein each of thesignals may also be described to have a channel, a carrier frequency, orassociated wavelength. The term “signal drift” is defined herein toinclude carrier frequency drift. Objective signal, also known in the artas primary signal, is defined herein to include radio frequency signalsthat carry data. A guard signal may also carry data. The term channeldescribes a wavelength. Power amplitude is defined herein as ameasurable strength of an electromagnetic signal. A measurable strengthincludes a magnitude of a signal. A measurable strength includesmeasuring an error rate. Error rate includes, for example, bit errorrate (BER) and/or signal-to-noise ratio (SNR). Error guard power isdefined as a measurable deviation of a measurable strength of a signalor a channel, wherein the measurable deviation is determined by amathematical comparison involving power amplitudes of signal(s) and/orchannel(s). The terms power amplitude, power, amplitude, and magnitudemay be used interchangeably. A channel is defined as a particularwavelength or a range of wavelengths. Since frequency is inverselyrelated to wavelength, features used to describe or associated withfrequency are also associated with wavelength. One of ordinary skill inthe art would recognize that the terms frequency and wavelength may beused equivalently and/or interchangeably. A Fourier transformed signalor waveform may be assumed to be substantially Gaussian shape.Accordingly, wavelength includes a wavelength of a signal defined to bethe wavelength at maximum power of a signal, or substantially near thewavelength at maximum power of the signal. Maximum power is the peakamplitude of a waveform. Substantially near is defined to be within onea deviation from the center of a substantially Gaussian shape.Accordingly, the description “substantially near the wavelength atmaximum power of the signal” includes a range of 26.

FIGS. 1-10 are illustrations of various embodiments and/or not-to-scaleexamples.

FIG. 1 shows an example of a wavelength-division multiplexed waveform 1.The WDM waveform 1 is generated by wavelength-division multiplexing twoobjective signals and two guard signals so that the WDM waveform 1 hasobjective signals at channels λo₁ and λo₂, and two guard signals atchannels, λ₁ and λ₂. A first multiplexed objective signal 101 and asecond multiplexed objective signal 102 are separated by a certain rangein wavelength, so that the two signals 101, 102 are immediate neighborsignals at different channels, λo_(l) and λo₂. A guard channel 103 isprovided between λo₁ and λo₂ of the first objective channel 101 and thesecond objective channel 102 defined as a general range of channels. Inthis example, the guard channel 103 includes two multiplexed guardsignals 104, 105 at channels λ₁ and λ₂, respectively. Accordingly, themultiplexed guard signals 104, 105 have channels that are between thewavelengths of the first multiplexed objective signal 101 and the secondmultiplexed objective signal 102. The waveform 1 illustrated in FIG. 1shows a state in which a significant signal drift is not detected, asneither of the multiplexed objective signals 101, 102 have drifted intothe guard channel 103 to measurably affect the multiplexed guard signals104, 105. Accordingly, an error rate of the multiplexed guard signals104, 105 would meet a satisfactory condition.

FIG. 2 shows an example of a wavelength-division multiplexed waveform 2similar to the waveform 1 shown in FIG. 1, except that the firstmultiplexed objective signal 101 has drifted to channel λ_(D1) so thatthe curvature of the signal's waveform, that indicates a measurableamplitude of the signal, has drifted into the guard channel 103. FIG. 2shows that the signal drift of the first multiplexed objective signal101 towards the channel of the first multiplexed guard signal 104measurably affects the multiplexed guard signal 104. For example, for anoptical signal, leakage of photons from the first multiplexed objectivesignal 101 to the first multiplexed guard signal 104 may cause adetectable and measurable increase in an error rate 110. The error rate110 may be determined by measuring a bit error rate (BER). Alternativelyor inclusively, the error rate 110 may be determined by measuring asignal-to-noise ratio (SNR). The error rate 110 may also be determinedby comparison of the power amplitude A_(g) of the outgoing guard signalto the power amplitude A′_(g) of the receiving multiplexed guard signal.In FIG. 2, the first multiplexed guard signal 104 has been illustratedto show the error rate 110 in a simplified manner to generally show thatthe first multiplexed guard signal 104 is somehow measurably affected bythe drifting first objective signal 101.

FIG. 3 shows another example of a wavelength-division multiplexedwaveform 3 similar to that shown in FIGS. 1 and 2, wherein a firstobjective signal 101, shown in FIGS. 1 and 2, has drifted further intothe guard channel 103 such that the first objective signal 101 hasoccupied the first multiplexed guard channel λ₁, generating a superposedsignal 106. The superposed signal 106 superposes the power of the firstobjective signal 101 with the first multiplexed guard signal 104, at thefirst multiplexed guard channel λ₁. The superposed signal 106 wouldcause a substantially detectable and measurable increase in an errorrate 112. Moreover, further drift of the first objective signal 101towards its immediate neighbor signal, the second multiplexed objectivesignal 102, although not shown, would be detected by an increase in theerror rate of the second multiplexed guard signal 105 with a reductionin the error rate at the first multiplexed guard channel λ₁. Thus, thedirection of the drift may be detected.

FIG. 4 shows another example of a wavelength-division multiplexedwaveform 4. The wavelength-division multiplexed waveform 4 is generatedby wavelength-division multiplexing two objective signals and four guardsignals. A first multiplexed objective signal 121 and a secondmultiplexed objective signal 122 are separated by a certain range inwavelength, so that the two signals 121, 122 are immediate neighborsignals at different channels or wavelengths. A guard channel 123 isprovided between the wavelengths of the first objective channel 121 andthe second objective channel 122 defined as a general range of channels.In this example, the guard channel 123 includes four multiplexed guardsignals 124, 125, 126, 127. Accordingly, the multiplexed guard signals124, 125, 126, 127 have channels that are between the wavelengths of thefirst multiplexed objective signal 121 and the second multiplexedobjective signal 122.

The guard channel 123 may include at least one action protocolassociated to at least one of the multiplexed guard signals 124, 125,126, 127. For example, an action protocol may include a predeterminedcriterion wherein if an error rate for the first multiplexed guardsignal 124 meets a predetermined threshold then an output is generated.For example, the output may be a recommendation for a system inspection.Preferably, an output recommending a system inspection includes when toperform the system inspection, such as within a certain time period(e.g. X hours, Y minutes) and/or a fixed determined time (year, month,date, hour, etc.). The action protocol may further include, for example,another predetermined criteria wherein if an error rate for the secondmultiplexed guard signal 125 meets a predetermined threshold then anoutput is generated wherein reporting a system malfunction alert thatcommunicates that an immediate attention to the system is requiredand/or recommend a system shutdown.

The waveform 4 illustrated in FIG. 4 shows a state in which asignificant signal drift is not detected, as neither of the multiplexedobjective signals 121, 122 have drifted into the guard channel 123 tomeasurably affect the multiplexed guard signals 124, 125, 126, 127.Accordingly, an error rate of the multiplexed guard signals 124, 125,126, 127 would meet a satisfactory condition. Thus, according to anaction protocol example, no output would be generated.

FIG. 5 shows an example of a wavelength-division multiplexed waveform 5similar to the waveform 4 shown in FIG. 4, except that the firstmultiplexed objective signal 121 has drifted into the guard channel 123.FIG. 5 shows that the signal drift of the first multiplexed objectivesignal 121 towards the channel of the first multiplexed guard signal 124measurably affects the multiplexed guard signal 124 to cause adetectable and measurable increase in an error rate 128. The error rate128 may be determined by measuring a bit error rate (BER), asignal-to-noise ratio (SNR), or a comparison of the power amplitudeA_(g) of the outgoing guard signal to the power amplitude A′_(g) of thereceiving multiplexed guard signal. The error rate 128 is illustrated ina simplified manner to generally show that the first multiplexed guardsignal 124 is somehow measurably affected by the drifting firstobjective signal 121. Moreover, for the detected signal drift shown inFIG. 5 and according to an action protocol example, an output may be arecommendation for a system inspection that may also include when toperform the system inspection.

FIG. 6 shows another example of a wavelength-division multiplexedwaveform 6 similar to that shown in FIGS. 4 and 5, wherein a firstobjective signal 121 has drifted further into the guard channel 123 suchthat the first objective signal 121 has occupied the second multiplexedguard channel λ₂. The occupation of the first objective signal 121 atthe second multiplexed guard channel λ₂ would cause a substantiallydetectable and measurable increase in an error rate 129. Further driftof the first objective signal 121 towards its immediate neighbor signal,the second multiplexed objective signal 122, would be detected by anincrease in the error rate of the third multiplexed guard signal 125with a reduction in the error rate of the first multiplexed guard signal124. Thus, the direction of the drift may be detected. Moreover, for thedetected signal drift shown in FIG. 6 and according to an actionprotocol example, an output may be a stronger recommendation for asystem inspection or an output indicating that a previously detectedproblem may be worsening.

FIGS. 4 to 6 and the associated detection and/or measurements of errorrates of the guard channel 123 would also indicate the direction of thesignal drift and which multiplexed objective signal is being affected bythe signal drift.

Further, if the error rate of the fourth multiplexed guard signal 127meets a certain condition and further the direction of the drift hasbeen detected that indicates that it is the first multiplexed objectivesignal that is drifting towards the second multiplexed objective signal,then according to an action protocol, an output reporting a systemmalfunction alert that communicates that an immediate attention to thesystem is required and/or recommend a system shutdown.

FIG. 7 illustrates an example of a waveform-division multiplexedwaveform 7 wherein between two multiplexed objective signals 131, 132, aguard channel 134 having a plurality of multiplexed guard signals areincluded. The guard channel 134 may include predetermined channelsλ_(i), wherein i=1 to N.

FIG. 8 illustrates an example of a waveform-division multiplexedwaveform 8 wherein between multiple multiplexed objective signals 141,142, 143, 144, multiple guard channels 146, 147, 148 having a pluralityof multiplexed guard signals are included. A guard channel is providedbetween each pair of the multiplexed objective signals. For example,FIG. 8 shows that the guard channel 146 is between multiplexed objectivesignals 141 and 142. The guard channel 147 is between multiplexedobjective signals 142 and 143. The guard channel 148 is betweenmultiplexed objective signals 143 and 144. Although FIG. 8 shows fourmultiplexed guard signals in each of the guard channels 146, 147, 148,the number of multiplexed guard signals in each of the guard channelsmay vary. Further, the guard channels may not necessarily include thesame uniform number of guard signals.

The number of guard signals in a guard channel may be one or more. Forexample, the multiplexed waveforms illustrated in FIGS. 1-8 showexamples wherein there are at least two guard signals per guard channel.However, using a device configured to generate a guard signal that canvary in wavelength, such as for example a tunable laser, a single guardsignal may use a channel hopping technique to cover multiple wavelengthsof a guard channel. Channel hopping is a technique wherein a singlesignal output device can vary the signal's wavelength among multiplechannels. An example of this method is illustrated in FIGS. 9A, 9B, and9C.

FIG. 9A shows an example of a wavelength-division multiplexed waveform9. The wavelength-division multiplexed waveform 9 is generated bywavelength-division multiplexing two objective signals and a guardsignal according to an example of a channel hopping protocol. A firstmultiplexed objective signal 151 is at channel λ_(o1) and a secondmultiplexed objective signal 152 is at channel λ_(o2). A multiplexedguard signal 153 is provided between λ_(o1) and λ_(o2) at wavelength λ₁.

FIG. 9B shows an example of a wavelength-division multiplexed waveform10. The wavelength-division multiplexed waveform 10 is generated bywavelength-division multiplexing two objective signals and a guardsignal according to an example of a channel hopping protocol. Amultiplexed guard signal 153 is provided between λ_(o1) and λ_(o2) atwavelength λ₂.

FIG. 9C shows an example of a wavelength-division multiplexed waveform11. The wavelength-division multiplexed waveform 11 is generated bywavelength-division multiplexing two objective signals and a guardsignal according to an example of a channel hopping protocol. Amultiplexed guard signal 153 is provided between λ_(o1) and λ_(o2) atwavelength λ₃.

An example of a channel hopping protocol includes a fixed sequence ofchanging the wavelength of the guard signal between wavelengths, such asλ₁ to λ₂ to λ₃ and back to λ₁ to repeat the sequence for a method usingthree guard signals in the guard channel. The number of guard signalsand wavelengths in the guard channel, and the specific sequence of thechange in wavelengths may be selected according to the needs of aparticular system. Other possible channel hopping protocols includevarying the guard wavelength λ_(g), from a predetermined set of guardchannel wavelengths λ₁ to λ_(N) (N≧2). Alternative channel hoppingprotocol includes varying the guard wavelength λ_(gi) from apredetermined set of guard channel wavelengths λ₁ to λ_(N) (N≧2)following a set pattern. Another channel hopping protocol includesselecting a guard wavelength λ_(gi) randomly from a predetermined set ofguard channel wavelengths λ₁ to λ_(N) (N≧2). The channel hoppingprotocol may include time variably selecting the second guard wavelengthλ_(gi) from the guard channel. Time variably selecting a wavelengthincludes a resulting wavelength from a calculation of a mathematicalequation that is a function of the time variable. Accordingly, more thanone tunable laser may be used in accordance to one or more channelhopping protocol(s) to cover a wide range of wavelengths within a guardchannel.

FIG. 10 shows a system for monitoring a WDM signal. The system comprisesa “Node 1” 202 that generates a first objective signal for carryingdata, “Node 2” 204 that generates a second objective signal for carryingdata, and a Guard Channel Monitor 206 that generates a guard signal. Thesystem also includes a Central Hub 210 configured to receive the firstobjective signal, the second objective signal, and the guard signal. Thefirst objective signal is sent from the “Node 1” 202 to the Central Hub210 along a communication pathway 222. The second objective signal issent from the “Node 2” 204 to the Central Hub 210 along a communicationpathway 224. The guard signal is sent from the Guard Channel Monitor 206to the Central Hub 210 along a communication pathway 226. The CentralHub 210 receives the signals to generate a multiplexed signal bywavelength-division multiplexing the first objective signal, the secondobjective signal, and the guard signal. The wavelength-divisionmultiplexed signal is sent to the guard Channel Monitor 206 along acommunication pathway 236. The wavelength-division multiplexed signalmay also be sent to “Node 1” 202 and “Node 2” 204 along communicationpathways 232 and 234, respectively. The Guard Channel Monitor 206 isconfigured to compare the guard signal to the wavelength-divisionmultiplexed signal at a predetermined wavelength. Comparing the guardsignal to the wavelength-division multiplexed signal includes, forexample, determining an error rate using methods of calculating BERand/or SNR. Preferably, the Central Hub 210 is a star coupler. Even morepreferably, the Central Hub 210 is a passive star coupler. Preferably,the communication pathways 222, 224, 226, 232, 234, 236 are opticalfibers. Preferably, the signals are optical signals.

Preferred embodiments have been described. Those skilled in the art willappreciate that various modifications and substitutions are possible,without departing from the scope of the invention as claimed anddisclosed, including the full scope of equivalents thereof.

What is claimed is:
 1. A method for monitoring a wavelength-divisionmultiplexed signal, comprising: generating from a first node, a firstoptical objective signal including a first objective wavelength λ_(o1),generating from a second node, a second optical objective signalincluding a second objective wavelength λ_(o2), and generating from amonitor, an optical guard signal including a guard wavelength λ_(g) thatis between λ_(o1) and λ_(o2); a central hub receiving the first opticalobjective signal, the second optical objective signal, and the opticalguard signal; the central hub generating the wavelength-divisionmultiplexed signal by wavelength-division multiplexing the first opticalobjective signal, the second optical objective signal, and the opticalguard signal, wherein the wavelength-division multiplexed signalincludes: a first multiplexed objective channel, a second multiplexedobjective channel, and a multiplexed guard channel having a range ofwavelengths between λ_(o1) and λ_(o2); receiving the wavelength-divisionmultiplexed signal by the monitor; and detecting an error of the firstmultiplexed objective channel by determining an error of the multiplexedguard channel, determining the error includes comparing the multiplexedguard channel to the optical guard signal.
 2. The method for monitoringa wavelength-division multiplexed signal according to claim 1, whereindetermining the error includes calculating a bit error rate (BER) of themultiplexed guard channel.
 3. The method for monitoring awavelength-division multiplexed signal according to claim 1, whereindetermining the error includes calculating a signal-to-noise ratio (SNR)of the multiplexed guard channel.
 4. The method for monitoring awavelength-division multiplexed signal according to claim 1, wherein thefirst optical objective signal further includes a first objective poweramplitude A_(o1); wherein the optical guard signal further includes apredetermined guard power amplitude A_(g); and wherein A_(g) is lessthan A_(o1).
 5. The method for monitoring a wavelength-divisionmultiplexed signal according to claim 4, wherein the predetermined guardpower amplitude A_(g) is less than or equal to 50% of A_(o1).
 6. Themethod for monitoring a wavelength-division multiplexed signal accordingto claim 1, wherein the optical guard signal includes a predeterminedguard power amplitude A_(g), the wavelength-division multiplexed signalfurther includes a multiplexed guard power amplitude A′_(g); and whereindetermining the error of the multiplexed guard channel includes:detecting the multiplexed guard power amplitude A′_(g), and determiningan error guard power ΔA_(g), wherein ΔA_(g)=f(A′_(g), A_(g)).
 7. Themethod for monitoring a wavelength-division multiplexed signal accordingto claim 1, further comprising: providing an action protocol associatedwith the multiplexed guard channel, wherein when the error of themultiplexed guard channel is determined to meet a criteria according tothe action protocol, an output is generated according to the actionprotocol.
 8. The method for monitoring a wavelength-division multiplexedsignal according to claim 7, wherein the output is a recommendation fora system inspection.
 9. The method for monitoring a wavelength-divisionmultiplexed signal according to claim 8, wherein the recommendation forsystem inspection includes when to perform the system inspection. 10.The method for monitoring a wavelength-division multiplexed signalaccording to claim 7, wherein the output is a system malfunction alert.